Liquid scintillation counting and automated instruments known as liquid scintillation counters are widely utilized to analyze samples containing radioactively labelled substances.
Typically, a sample in solution is mixed with a liquid scintillator, commonly referred to as a cocktail, and the light events produced from the sample and cocktail mixture are detected according to their energy and number of events. The light events occur when the energy of the particles, emitted from the radioactive isotope component of the sample in solution, is transferred to the molecules of the liquid scintillator. This produces a light emission of a specific energy range which is characteristic of the radioactive isotope.
Detecting both the energy and number of light events in a particular energy range provides the information necessary to construct a spectrum. Using this information the radioactive species can be quantitatively analyzed. Liquid scintillation counting and automated instruments to perform liquid scintillation counting have been widely discussed in a multitude of publications and patents.
Scintillation counting of liquid samples has certain disadvantages attributable to the nature of the liquid species used. One is a phenomenon known as quench. Quench commonly refers to a chemical or optical effect on the scintillation process which results in loss of light events or reduction in light emission energy. The chemical nature of the solution in which the sample and scintillator are mixed and the color of the liquid sample solution are the causative agents. The result is inefficiency in the ability of the liquid scintillation counter to accurately count the scintillations resulting from particle disintegrations of the isotopes, and therefore interference with sample analysis.
Another disadvantage in scintillation counting of liquid samples is that, after analysis, the liquid produced by mixing the radioactive sample with the cocktail must be disposed of. The regulations governing the disposal of liquid radioactive materials are particularly rigorous. Due to the volume of liquid radioactive materials that require disposal, the costs can be considerable. In many cases a solid material having a radioactive nature is easier to dispose of and incurs far less expense.
The technique or solid scintillation counting (SSC) differs from liquid scintillation counting (LSC) in a very fundamental way. In SSC the sample is presented as a liquid and then by evaporation/drying is converted to a solid residue which ends up in intimate contact with the solid scintillator. In LSC the sample is presented as a liquid and counted (analyzed) in the same liquid form. Thus SSC can be seen to have advantages over LSC in that significantly smaller volumes of waste are generated. The present solid scintillators are inorganic phosphors (e.g. yttrium silicate doped with cerium, zinc sulphide doped with silver, yttrium oxide doped with europium, etc.) and as such are not combustible.
At present there are four types of scintillation compositions. These are:
1. solid scintillators comprising a crystal of a solid hydrocarbon material; PA1 2. liquid scintillators which comprise one or more suitable solid scintillators dissolved in a liquid solvent; PA1 3. so-called solid solution scintillators which comprise a solid scintillator in a solid polymeric solution; and PA1 4. solid scintillators comprising a crystal of a suitable inorganic material.
The present invention is not readily categorized by the above groups but rather seems to require a new grouping of "dry" liquid scintillators.
The following patents further illustrate the uses of solid scintillators and the technique of solid scintillation counting.
European Patent EP 0 212 450 A1, March 1987, describes a dry solid scintillator counting composition for the detection of radioactive substances in a liquid comprising a mixture of fluor particles and a binder bonding the particles into a coherent structure.
U.S. Pat. No. 4,562,158, Dec. 31, 1985 describes a solid phase scintillation counting method wherein a scintillating material is added to an inert carrier and a radioactive substance is caused to contact the carrier either before or after the scintillating material has been applied.
International Patent application WO 89/02088 & 9 describes the use of an inorganic solid scintillator which is attached to a solid support medium by a binder material.
U.S. Pat. No. 4,127,499, Nov. 28, 1978 describes scintillation counting compositions comprising polymeric particles derived from a latex and loaded with at least one uniformly dispersed hydrophobic fluor so as to permit detection of low-energy radiation.
U.S. Pat. No. 3,491,235, Jan. 20, 1970 describes a method for producing fluorescent layers by dispersing an organic solution of fluorescent compounds in aqueous colloid solution, coating and drying.
Japanese Patent Publication Sho 63-101787, May 6, 1988 describes multi-layer scintillators made by piling up either mixed monomolecular films consisting of radiation absorbing compounds and compounds emitting ultraviolet, visible or infrared radiation, or monomolecular films consisting of radiation absorbing compounds and separate monomolecular films consisting of compounds emitting ultraviolet, visible or infrared radiation. The layers are deposited from a solution of the compounds in chloroform.
U.S. Pat. No. 4,588,698, May 13, 1986 describes the microencapsulation of solid phase scintillators in gels which are selectively permeable to a diffusible radioactive label.
All of the aforementioned types of solid scintillators have limitations on their use which although not totally inhibitory nevertheless impose limitations on the scope of their use. The solid scintillators comprising a crystal of a solid hydrocarbon material are limited in use when the radioactively labelled substance is presented in an organic solvent medium. The so-called solid solution scintillators are limited by their inability to achieve intimate contact between the emitter and the fluor. Such intimate contact is particularly important when weak, short range radiations are to be detected by the fluor, for example, sample systems of weak beta-emitters such as tritium and Carbon-14 or gamma-emitters such as iodine-125. Liquid scintillation counting compositions are capable of detection efficiencies above 35% and in some cases theoretically 100%, most probably due to the intimate radioactive emitter-fluor contact possible in a liquid medium.
As will be seen from the following description, the present invention represents a significant improvement over the prior art methods for counting scintillations resulting from radioactive emissions in a sample for analysis.